<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<style>

body {
  height: 50px;
  -moz-column-count: 2;
}
.container {
  height: 10px;
}
.overflow {
  height: 100px;
}
.bb {
  border-bottom: solid 4px magenta;
}

</style>

<script>

function boom()
{
  document.getElementById("x").setAttribute("class", "");
  document.getElementById("y").setAttribute("class", "bb");
}

</script>
</head>

<body onload="boom();">
  <div class="container"><div class="overflow" id="x"></div></div>
  <div id="y"></div>
  <div class="container"><div class="overflow"></div></div>
</body>
  
</html>
